1. Field of the Invention
The present invention relates to a developer for developing a magnetic latent image and a magnetic image formation apparatus using the same.
2. Related Arts
FIG. 4 is a view showing a conventional wet magnetic image formation apparatus 80. The image formation apparatus 80 comprises a magnetic drum 81, a magnetic head 82, a developing device 83 and a transfer roller 84. The magnetic drum 81 has a cylindrical shape and is provided with the magnetic head 82, the developing device 83 and the transfer roller 84 in this order along a direction of rotation 89 on an outer periphery of the magnetic drum 81. Operation of the magnetic drum 81 is synchronized with operations of the magnetic head 82, the developing device 83 and the transfer roller 84. In accordance with rotation of the magnetic drum 81, a magnetic image is formed as mentioned below. Image data from an information instrument (not shown) are sent to the magnetic head 82 and recorded on the magnetic drum 81 as a magnetic latent image by the magnetic head 82.
The developing device 83 stores a developer 94. The developer 94 is composed of a magnetic powder of a ferromagnetic pigment and a liquid ink consisting of a solvent and a coloring pigment or a dye. Specific gravity of the magnetic powder is larger than that of the liquid ink, so that the magnetic powder cannot be kept dispersed uniformly in the liquid ink for a long time by itself. In order to disperse the magnetic powder uniformly in the liquid ink to form the developer 94 having uniform magnetic characteristics, a surface active agent, a resin or the like is added to the liquid ink.
FIG. 5 is a view showing a state of a magnetic powder 91 when a surface active agent 92 is added thereto. When the surface active agent 92 is added, the surface active agent 92 binds to the periphery of the magnetic powder 91. FIG. 6 is a view showing a state of the developer 94 when the surface active agent 92 is added. By using the surface active agent 92, the magnetic powder 91 can disperse uniformly in a liquid ink 93.
Referring to FIG. 4 again, by means of a developing roller 88 provided in the developing device 83, the developer 94 is supplied to a peripheral surface of the magnetic drum 81, whereby the magnetic latent image is developed. By the transfer roller 84, the latent image formed by the developer 94 on the magnetic drum 81 is transferred to a recording paper 87. Finally, the magnetic latent image on the magnetic drum 81 is erased and the developer 94 remaining on the magnetic drum 81 is removed. By repeating these, images are formed successively. Such a conventional wet magnetic image formation apparatus is disclosed for example, in Japanese Unexamined Patent Publication No. Hei 5(1993)-188827.
As for a magnetic image formation apparatus, there are dry magnetic image formation apparatuses 40 and 50 besides the wet magnetic image formation apparatus 80. FIG. 7 is a view showing the dry magnetic image formation apparatus 40. Though a process for forming an image for the dry magnetic image formation apparatus 40 is the same as the one for the above-mentioned wet magnetic image formation apparatus 80, methods for developing and transferring an image for the dry magnetic image formation apparatus 40 are different from the ones for the wet magnetic image formation apparatus 80. A dry toner to be used for the dry magnetic image formation apparatus 40 is pretreated to have a sensitivity to both magnetic force and electrostatic force. Accordingly, the toner adheres to the magnetic latent image formed on the magnetic drum 41 in correspondence with the magnetic force of the magnetic latent image and therefore, an image is developed. Moreover, by the transfer roller 43 facing the magnetic drum 41 across a recording paper 44, an electrostatic force larger than the above-mentioned magnetic force is applied to the toner on the magnetic drum 41. Accordingly, the toner is attracted towards the transfer roller 43 and is transferred to a surface of the recording paper 44. Then, heat pressing by a fixing roller 45 is applied to the toner image transferred on the recording paper 44 to fix it. In addition, Japanese Unexamined Patent Publication No. Sho 63(1988)-300275 discloses a wet magnetic image formation apparatus for forming an image by the same image formation process as the one for the dry magnetic image formation apparatus 40, wherein a liquid toner has a sensitivity to both magnetic force and electrostatic force.
FIG. 8 is a view showing another dry magnetic image formation apparatus 50. Process in the dry magnetic image formation apparatus 50 and the dry magnetic image formation apparatus 40 are the same until the developing step is performed. As for the dry magnetic image formation apparatus 50, by applying pressure to a magnetic drum 51 by a transfer roller 53, a toner image on the magnetic drum 51 is transferred to a recording paper 55 and fixed at the same time and further, securely fixed by a pressing device 56.
The two representative dry magnetic image formation apparatuses 40 and 50 mentioned above have problems as follows. In the former case, it is difficult to allow each toner particle of the toner to have the same electromagnetic characteristics, the toner having a sensitivity to both magnetic force and electrostatic force. Therefore, it is impossible to perform a stable image formation at all times. This situation remains the same in the case of the wet magnetic image formation apparatus disclosed in Japanese Unexamined Patent Publication No. Sho 63(1988)-300275. In the latter case, since transferring and fixing are performed at the same time by applying a high pressure of about 10 kg to contact-bond a toner image on a recording paper 55, a pressing mechanism 57 in the apparatus containing the transfer roller 53 and the pressing device 56 is complicated and therefore, an apparatus itself becomes expensive. Moreover, by applying a high pressure on the toner, the toner image transferred to the recording paper 55 becomes glossy and unattractive, and moreover, the recording paper 55 is apt to be wrinkled. Further, if the recording paper 55 has a low smoothness, fixation of the toner is so weak that the toner is apt to come off.
In the wet magnetic image formation apparatus 80, transferring is performed by contacting an ink image formed on the magnetic drum 81 with a recording paper 87 with low pressure. Accordingly, the problems which the dry magnetic image formation apparatuses 40 and 50 have can be solved by using the wet magnetic image formation apparatus 80. However, in the wet magnetic image formation apparatus 80, when the liquid ink 93 is transferred to the recording paper 87, not only the liquid ink 93 but also the magnetic powder 91 is transferred to the recording paper 87 because of a strong binding force of the surface active agent 92. As mentioned above, since the magnetic powder 91 is a pigment, the image formed on the recording paper 87 becomes a peculiarly colored and blurred image, so that full reproducibility of colors cannot be achieved.